An electrophotographic light-sensitive material may have various structures depending on the characteristics required or an electrophotographic process to be employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Further, a process of using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced.
Binders which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to have film-forming properties by themselves and the capability if dispersing a photoconductive powder therein. Also, the photoconductive layer formed using the binder should have satisfactory adhesion to a base material or support. The photoconductive layer formed by using the binder also must have various electrostatic characteristics and image-forming properties, such that the photoconductive layer exhibits high charging capacity, small dark decay and large light decay, hardly undergoes fatigue before exposure, and maintains these characteristics in a stable manner against change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (see JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese patent application"), styrene-butadiene resins (see JP-B-35-1960), alkyd resins, maleic acid resins, and polyamide (see JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946), etc. However, electrophotographic light-sensitive materials using these known resins have a number of disadvantages, i.e., poor affinity for a photoconductive powder (poor dispersion of a photoconductive coating composition); low photoconductive layer charging properties; poor reproduced image quality, particularly dot reproducibility or resolving power; susceptibility of the reproduced image quality to influences from the environment at the time of electrophotographic image formation, such as high temperature and high humidity conditions or low temperature and low humidity conditions; and insufficient film strength or adhesion of the photoconductive layer, which causes, when the light-sensitive material is used for an offset master, peeling of the photoconductive layer during offset printing thus failing to obtain a large number of prints; and the like.
To improve the electrostatic characteristics of a photoconductive layer, various approaches have hitherto been taken. For example, incorporation of a compound containing an aromatic ring or furan ring containing a carboxyl group or nitro group either alone or in combination with a dicarboxylic acid anhydride into a photoconductive layer has been proposed as disclosed in JP-B-42-6878 and JP-B-45-3073. However, the thus improved electrophotographic light-sensitive materials still have insufficient electrostatic characteristics, particularly light decay characteristics. The insufficient sensitivity of these light-sensitive materials has been compensated for by incorporating a large quantity of a sensitizing dye into the photoconductive layer. However, light-sensitive materials containing a large quantity of a sensitizing dye undergo considerable deterioration of whiteness to reduce the quality as a recording medium, sometimes causing a deterioration in dark decay characteristics, resulting in a failure to obtain a satisfactory reproduced image.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") suggests control of the average molecular weight of a resin to be used as a binder of the photoconductive layer. According to this suggestion, the combined use of an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 and an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.4 to 2.times.10.sup.5 would improve the electrostatic characteristics (particularly reproducibility as a PPC light-sensitive material on repeated use), moisture resistance, and the like.
In the field of lithographic printing plate precursors, extensive studies have been conducted to provide binder resins for a photoconductive layer having electrostatic characteristics compatible with printing characteristics. Examples of binder resins so far reported to be effective for oil-desensitization of a photoconductive layer include a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point of from 10.degree. C. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit with a substituent having a carboxyl group at least 7 atoms distant from the ester linkage as disclosed in JP-A-53-54027; a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544; and a terpolymer containing a (meth)acrylic ester unit with an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046.
However, none of these resins proposed has proved to be satisfactory for practical use in charging properties, dark charge retention, photosensitivity, and surface smoothness of the photoconductive layer.
The binder resins proposed for use in electrophotographic lithographic printing plate precursors were also proved by actual evaluations to give rise to problems relating to electrostatic characteristics and background staining of prints.
In order to solve these problems, it has been proposed to use, as a binder resin, a low-molecular weight resin (molecular weight: 1.times.10.sup.3 to 1.times.10.sup.4) containing from 0.05 to 10% by weight of a copolymer component having an acid group in the side chain thereof to thereby improve surface smoothness and electrostatic characteristics of the photoconductive layer and to obtain background stain-free images as disclosed in JP-A-63-217354. It has also been proposed to use such a low-molecular weight resin in combination with a high-molecular weight resin (molecular weight: 1.times.10.sup.4 or more) to thereby obtain sufficient film strength of the photoconductive layer to improve printing durability without impairing the above-described favorable characteristics as disclosed in JP-A-64-564, JP-A-63-220148 and JP-A-63-220149
It has turned out, however, that use of these resins is still insufficient for stably maintaining performance properties in cases when the environmental conditions greatly change from high-temperature and high-humidity conditions to low-temperature and low-humidity conditions. In particular, in a scanning exposure system using a semi-conductor laser beam, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light and, hence, higher performance with respect to electrostatic characteristics, and particularly dark charge retention and photosensitivity has been demanded.